Week10
•  Lab3!
–  Photometricquality.Stampoutthosebad
points.Finishit.
•  Lab4!
–  Greatdata.Eveningsessionsthisweek
focusonLab3wrap-upandLab4
reducGons.
•  Examsreadyforreturn
•  Readthebook!ChromeyChapters9
and10areexcellentreinforcement.
–  Don’tplanongoingtograduateschool
without“owning”thisinformaGon.
•  Finalproject–familiarizeyourselfwith
theavailableinstrumentsatAPO,
browsethescheduleforproposal
abstracts.
–  hVp://www.apo.nmsu.edu
http://www.skyandtelescope.com/astronomy-news/observing-news/anotherimpact-on-jupiter-032920161/
Topics:
•
CCD/Detector/Calibration wrapup.
•
At-the-telescope techniques wrapup.
•
Aperture photometry
•
PSF fit photometry
CCDOverscan
•  SincethenumberofshiYsappliedtoaCCDduringthereadout
phaseisarbitraryitispossibleto
–  Underscan–readoutonlyafracGonofthepixels(foraCCDlargelyinone
corner).DoingsocouldreducereadoutGmebetweenframessubstanGally.
–  Overscan–clockmorepixelsthanareactuallythere.Doingsobrings
“empty”fakepixelstothereadoutamplifier.Thesefakepixelsareusefulfor
determiningGmedependentbiaschangessincetheyarereadatnearlythe
sameGmeasthepixelsinagivenrow.
Chromey
Electrical “pickup”
InfraredArrays
•  VisiblewavelengthCCDsbenefitfromthefactthattheprimary
integratedcircuitconstrucGonmaterialissilicon,whichalsoserves
asthedetectormaterial.
•  Infrareddetectorsrequirematerialwithsmallerbandgap(InGaAs,
InSB,HgCdTearepopular)whichmustbebondedtothereadout
electronics(sGlloutofnecessitymadeofsilicon).
InfraredArrays
•  Thisarchitecturenaturallyleadstodesigningreadoutcircuitswhich
havereadoutamplifiersunderEVERYpixel.
–  Thechargenevermoves.Pixelsare“addressed”bytheir(x,y)coordinates.
–  ThecollecGngimagecanbereadoutwithoutdisturbingit(non-destrucGve
readout).
–  VeryeasytoaddressmulGpleandrandomlydistributedsubapertures.
–  Easytoestablishguideareaswhiletherestofthechipintegrates.
InfraredArrays
•  Astrophysicaladvantages
–  SensiGvitytocoolerobjects.
•  Weinpeakisat500nminthevisiblefor6000Kobjects.
–  TheSunand“hot”stars
•  Weinpeakisat2000nm(2microns)intheinfraredfor1500Kobjects.
–  Formingstars,selfluminousyoungmassiveplanets,browndwarfs…
•  Weinpeakisat100,000nm(100microns)inthefarinfraredfor30K
objects.
–  “Warm”dustingalaxies,masslosingstars
InfraredArrays
•  Astrophysicaladvantages
–  SensiGvitytocoolerobjects.
•  Weinpeakisat500nminthevisiblefor6000Kobjects.
–  TheSunand“hot”stars
•  Weinpeakisat2000nm(2microns)intheinfraredfor1500Kobjects.
–  Formingstars,selfluminousyoungmassiveplanets,browndwarfs…
•  Weinpeakisat100,000nm(100microns)inthefarinfraredfor30K
objects.
–  “Warm”dustingalaxies,masslosingstars
InfraredArrays
•  Astrophysicaladvantages
–  SensiGvitytocoolerobjects.
•  Weinpeakisat500nminthevisiblefor6000Kobjects.
–  TheSunand“hot”stars
•  Weinpeakisat2000nm(2microns)intheinfraredfor1500Kobjects.
–  Formingstars,selfluminousyoungmassiveplanets,browndwarfs…
•  Weinpeakisat100,000nm(100microns)inthefarinfraredfor30K
objects.
–  “Warm”dustingalaxies,masslosingstars
InfraredArrays
•  Astrophysicaladvantages
–  Increasingtransparencyofdustatlongerwavelengths.
•  Interstellargrainsaretypicallysub-microninsizeandgoodatblocking
wavelengthsoflighttheirsizeorsmaller.Red/infraredgetsthrough,
blueblocked.
InfraredArrays
•  Astrophysicaladvantages
–  Increasingtransparencyofdustatlongerwavelengths.
•  Interstellargrainsaretypicallysub-microninsizeandgoodatblocking
wavelengthsoflighttheirsizeorsmaller.Red/infraredgetsthrough,
blueblocked.
Focusandf/#
•  The“speed”ofanopGcalsystemdeterminesitssensiGvitytofocus
changes(specificallydefocusasafuncGonofdisplacementofthe
sensorfocalplanerelaGvetotheopGcstruefocalplane).
–  “Fast”systemshavelowf/#’s.“Slow”systemshavehighf/#’s.
•  Thesizeofthedefocusedimageistheupstreamordownstream
displacementofthefocalplanedividedbythef/#.
FocusinganOpGcalSystem
•  Mosttelescopeshaveaphysicalknobtoturnoranelectronic
controlofprimaryvs.secondaryseparaGontoalterthelocaGonof
thefocalplane.
•  InsimplesttermsfocusinginvolvesminimizingtheFWHMorRMS
sizeofthefocalplaneimagesofpointsources.
–  Theimagesizeminimumcanbedictatedbymanyfactors
•  Airydiscsize/diffracGonlimit
•  SystemopGcs
•  Camerapixelsize
•  Seeing
FocusinganOpGcalSystem
•  Mosttelescopeshaveaphysicalknobtoturnoranelectronic
controlofprimaryvs.secondaryseparaGontoalterthelocaGonof
thefocalplane.
•  InsimplesttermsfocusinginvolvesminimizingtheFWHMorRMS
sizeofthefocalplaneimagesofpointsources.
–  Theimagesizeminimumcanbedictatedbymanyfactors
•  Airydiscsize/diffracGonlimit
•  SystemopGcs
•  Camerapixelsize
•  Seeing
Extreme defocus shows you the telescope entrance pupil.
SeekingOpGmalFocus
•  BecausefindingfocusinvolveslocaGngtheminimumofafuncGon,“eyeballing”
focuscanbefrustraGngandGmeconsuming,especiallywithvariableseeing.
•  BeVertotakeFWHMmeasurementsvs.encodedfocusvaluesystemaGcallyand
fittheminimum.
–  Or…“findthewings,splitthedifference”
Focusvs.Temperature
•  Metalscontractastemperaturedecreases.
•  Sinceametalstructureestablishestheprimary/secondaryseparaGon
inaCassegraintelescopeortheobjecGve/eyepieceseparaGonina
classicalrefractoropGmalfocusistemperaturedependent.
TrackingandAlignment
•  AtelescopecannotnecessarilybeleYonitsowntotrackthestars
–  TheR.A.driveratecanbeslightlyoff
–  Thegeartraincanbeimperfect
–  Thepolaraxiscanbemisaligned
Guiding/Autoguiding
•  WithrealGmeimagery,eitherfromasub-regionofthe“science”
arrayorfromaseparateguidingarray,trackingerrorscanbe
minimizedwithsmallguidingcorrecGons(buVonpushes)appliedto
thetelescopedrives.
http://www.celestron.com/browse-shop/astronomy/mounts-and-tripods/cge-pro-mount
HandPaddles
•  Manualcontroloftelescopesoccursbothatcontrolroomconsoles,
oncomputerscreensand,inthedome,withamulG-buVonhand
paddle.
HandPaddles
•  Manualcontroloftelescopesoccursbothatcontrolroomconsoles,
oncomputerscreensand,inthedome,withamulG-buVonhand
paddle.
AperturePhotometry
•  Asinglepixelinastar’simagecontainsfluxfromthestarand
backgroundfromtheskyandpotenGallyfromunwantedcelesGal
sources.
AperturePhotometry
•  Measuretheskyinanannulussurroundingthestar,hopingthatthe
annulusmedian(notaverage!)fluxisrepresentaGveofthe
backgroundunderlyingthetarget.
AperturePhotometry
•  Addupthefluxinthe(red)staraperturesubtracGngoffthe
esGmateoftheskyfluxfromthe(green)skyannulus.
AperturePhotometry–Decisions,Decisions…
•  Howbigshouldthestaraperturebe?
–  ToobigandyougetextraskyfluxandassociatedPoissonnose(nottomenGon
contaminaGngunwantedstars).
–  ToosmallandyoucaptureonlyafracGonofthestar’sflux.
•  Whereshouldtheinnerandouterskyannulusradiibe?
–  InnerradiustooGghtandtheskyannuluswillcontainstellarfluxbiasingthe
background.
–  OuterradiustoolargeandthemeasuredskymaybeunrepresentaGveofwhat
underliesthetarget.
•  Whattorejectandhow?
–  Theskyannulusintheexamplecontainsabrightstar.
•  Usea“cookiecuVer”toremoveitfromtheaccounGng?
•  RejectitsfluxstaGsGcally,forexampleusingamedianratherthanan
average.
•  RejectpixelslyingmorethanNstandarddeviaGonsabovethemeaninthe
skyannulus.
EffectsofVaryingtheStarAperture
•
•
•
Theencircledfluxfromthestar
increaseswithincreasing
primaryapertureunGlthe
aperturecapturesmostallof
theflux.
Aslongasthesameapertureis
usedonthetargetand
comparisonstarsthemagnitude
differencewillbecorrectevenif
notallofthefluxisinthe
aperture.
Infactitmaybepreferableto
useanaperturethatthatisnot
overlylargetoavoid
contaminaGonfrom
backgroundandotherstars.
EffectsofEstablishingtheCentroid
•  PhotometricresultsdependonthecenteroftheextracGonaperture.
–  SlightshiYswillchange,fracGonally,whatpixelsarein/outoftheaperture.
∑∑ x * (Image[x, y]− Background)
xcen =
x
y
∑∑
x
(Image[x, y]− Background)
y
Chromey p. 311
•  Sincesincetheaperturesumusedtogetthestellar(aswellas
backgroundannulus)fluxusespixelsandfracGonalpixelsbasedon
thecentroidtheresultswillchangeslightlywithslightshiYsin
centroid.
CentroidNoise
•  Inhigh-precisionphotometrycentroidnoisemaybe
thelimiGngfactor.
–  ArobustcentroidingalgorithmisessenGal.
–  Consideraseriesofmeasurmentswithsmallsub-pixel
centroidoffsets.
https://mirametrics.com/tech_note_centroid_noise.htm
PSF-fitPhotometry
•  IndensefieldsaperturephotometrymaybecomeimpracGcal.
•  Inevitablyaperturephotometryalsoaccommodatesmore
backgroundthandesirable.
–  Extractstellarfluxintheleanestmeanestfootprintpossible….
PSF-fitPhotometry
•  Thepeakcountsforastarcanbeagoodproxyforitsflux.
–  BeVeryet,thefullstellarprofile/shapeprovidesinformaGonthatcanyieldan
opGmalesGmateofthetotalfluxusingleastsquarefiongprocedureswhere
thecentroidandpeakarethefreeparameters.
PSF-fitPhotometry
•  Thepeakcountsforastarcanbeagoodproxyforitsflux.
–  Determinethestellarprofile(PSF)fromallofthestarsontheframe.
–  IntheabsenceofsignificantopGcaldistorGonsallstarsonanimagehavethe
sameshape.Fitthatshapetoeachstarimage.
–  Fitthebrightstarsfirstandsubtract,leavingacleanerimageonwhichtofit
thefainterstars.